Material for negative electrode of non-aqueous electrolyte secondary battery, process for producing the same, negative electrode and battery

ABSTRACT

A negative electrode material for non-aqueous electrolyte secondary batteries, comprises: a carbon material having a sphericity of at least 0.8, and exhibiting an average (002) interlayer spacing d 002  of 0.365-0.400 nm, a crystallite size in a c-axis direction Lc (002)  of 1.0-3.0 nm, as measured by X-ray diffractometry, a hydrogen-to-carbon atomic ratio (H/C) of at most 0.1 as measured by elementary analysis, and an average particle size Dv 50  of 1-20 μm. The negative electrode material is spherical and exhibits excellent performances including high output performance and durability.

TECHNICAL FIELD

The present invention relates to an electrode of secondary batteries andmore particularly to a negative electrode material for non-aqueouselectrolyte secondary batteries which allows quick doping-dedoping ofbattery active substance and is excellent in charge-discharge cyclecharacteristic.

BACKGROUND ART

As a type of high-energy density secondary battery, there has beenproposed a non-aqueous electrolyte-type lithium secondary battery (e.g.,Patent documents 1-4 listed below). The battery utilizes a phenomenonthat a carbon intercalation compound of lithium can be easily formedelectrochemically, and when the battery is charged, lithium in thepositive electrode comprising, e.g., a chalcogenide compound such asLiCoO₂, is electrochemically inserted between carbon layers in thenegative electrode (doping). The carbon thus-doped with lithium functionas a lithium electrode to cause a discharge, whereby the lithium isliberated (dedoped) from the carbon layers to return to the positiveelectrode.

In order to achieve a higher energy density in such a non-aqueouselectrolyte-type lithium secondary battery, it is necessary to increasethe amount of lithium dedoped and doped per unit weight of the positiveelectrode, substance and the amount of lithium doped and dedoped perunit weight of the negative electrode. From such a viewpoint, agraphitic material having high doping and dedoping capacity per volumehas been used particularly as a negative electrode material.

In recent years, a non-aqueous electrolyte-type lithium secondarybattery has been expected to be used not only as a power supply forsmall-size portable instruments but also as a power supply for a hybridelectrical vehicle (hereinafter abbreviated as a “HEV”). Such a HEV isloaded with an internal-combustion engine in addition to the battery asmotive power supplies therefor, so that the battery is not required tosupply a large amount of energy but is required to supply a high poweroutput capable of driving the vehicle or sufficiently supplementing themotive power of the vehicle. Further, in order to achieve a lower fuelconsumption, it is indispensable to effectively recover a braking energyof the vehicle and is further required to exhibit a high input capacity.

On the other hand, while the expected life of a non-aqueous electrolytesecondary battery as a power supply for small-size portable instrumentsis several years, a power supply system for HEVs, comprising severalhundreds of batteries connected in series cannot be easily exchanged inthe middle of the life of the vehicle but is required to exhibit a lifeand a reliability comparable to the life of the vehicle, i. e., of 10 ormore years.

As a means for improving the output performance of a non-aqueouselectrolyte secondary battery, there has been proposed to control theelectrode thickness and the particle size of the active substance(Patent document 5 listed below). More specifically, by making thinnerthe electrode, it becomes possible to increase the reaction area andreduce the reaction potential difference in the electrode thicknessdirection. As a result, it becomes possible to reduce a polarizationbetween the surfacemost layer and a layer close to the electroconductivesubstrate of the electrode, thereby reducing a lowering of performanceat the time of a large current discharge. However, the output is not yetsufficient and a higher output is demanded. Further, accompanying theuse of a thinner electrode, larger numbers of conductive substrates andseparators for the positive and negative electrodes are required thanusual, and this results in a lowering of energy density of the battery,for which an improvement is also desired.

As for the reliability of a negative electrode material, a graphiticmaterial and a graphitizable carbon material having a turbostratictexture are liable to cause a repetition of expansion and constrictionof crystallites at the time of doping and dedoping of lithium, so thatthey are poor in reliability as a negative electrode material ofnon-aqueous electrolyte secondary battery used for HEVs. On the otherhand, non-graphitizable carbon material causes little expansion andconstriction at the time of doping and dedoping of lithium to exhibit ahigh cycle durability so that it is expected to be promising as anegative electrode material of non-aqueous electrolyte-type lithiumsecondary battery used for HEVs. However, the texture ofnon-graphitizable carbon is variously changed depending on the textureof a carbon precursor and heat-treatment conditions thereafter, anappropriate texture control is important for achieving goodcharge-discharge performances. Non-graphitizable carbon particleexhibiting good charge-discharge performances have been obtained throughpulverization of a carbon precursor itself or after calcination thereof,so that it requires a lot of pulverization energy for providing asmaller particle size which is indispensable for achieving a thin layerof electrode active substance and the smaller particle size isaccompanied with an increased amount of fine powder to result in alowering in reliability of the battery. There also arises a problem thatthe enhancement of pulverization and removal of fine particles forproviding the smaller particle size results in a remarkable lowering inpulverization efficiency.

It has been proposed to use a non-graphitizable carbon having aspherical shape as a negative electrode active substance for providing anon-aqueous electrolyte exhibiting a high energy density and lessliability of short circuit due to formation of dendrite, thus exhibitinga high reliability (Patent document 6 listed below). It is intended toform a negative electrode having a uniform distribution of activesubstance through coating, etc., by using spherical carbon as thenegative electrode active substance, and thereby to provide a negativeelectrode with less liability of internal short circuit due to dendriteformation and with an electrical capacity closer to a theoretical one.However, substantially no process for producing the sphericalnon-graphitizable carbon is disclosed, and the description of a bulkspecific gravity of 0.6-0.9 therein provides an assumption that thecarbon had a true density of ca. 1.7-2.5, which rather falls within aregion of from graphitizable carbon to graphite. Further, the dischargecapacity thereof was at most 320 mAh/g, which does not exceed thetheoretical capacity of graphitic material and is not sufficientlylarge.

On the other hand, while it is easily conceived of carbonizing aspherical synthetic resin in order to obtain a spherical carbonmaterial, this is actually not easy. Synthetic reins include:thermosetting resins causing polycondensation under heating and vinylresins obtained through radical polymerization. A thermosetting resingenerally provides a relatively good carbonization yield, but it forms aviscous condensate difficult to handle at an initial stage ofcondensation and requires further many steps for sphering thereof. Aspherical non-graphitizable carbon obtained from phenolic resin as thestarting material is disclosed in Patent document 7 listed below, whichhowever does not disclose a process for producing spherical phenolicresin as the starting material. Further, the resultant sphericalnon-graphitizable carbon exhibited a fairly low discharge capacity of185 mAh/g. On the other hand, vinyl resins can be obtained as sphericalpolymerizates through radical suspension polymerization, but most ofthem cause de-polymerization or thermal decomposition at the time ofcarbonization treatment, thus failing to leave a substantial amount ofcarbonization product. Further while the carbonization of a syntheticresin generally results in a non-graphitizable carbon, but such anon-graphitizable carbon is liable to be oxidized when left in the air,thus resulting in an increase of irreversible capacity. This difficultyhas been obviated by storing the non-graphitizable carbon or anelectrode obtained by using it in a layer of negative electrode activesubstrate in a non-oxidizing gas atmosphere, which however has posed asubstantial problem in the production process.

-   Patent document 1: JP-A 57-208079-   Patent document 2: JP-A 62-90863-   Patent document 3: JP-A 62-122066-   Patent document 4: JP-A 2-66856-   Patent document 5: JP-A 11-185821-   Patent document 6: JP-A 6-150927-   Patent document 7: JP-A 6-20680

DISCLOSURE OF INVENTION

In view of the above-mentioned problems of the conventional materials,the present invention aims at providing a negative electrode materialfor non-aqueous electrolyte secondary battery which has a high outputperformance, a high durability and a high discharge capacity, a processfor production thereof, and an electrode and a non-aqueous electrolytesecondary battery comprising the negative electrode material.

In the course of the present inventors' study for producing a negativeelectrode material for a non-aqueous electrolyte secondary batteryexhibiting good performances as a power supply for HEVs, it has beenfound that a spherical carbon material obtained by subjecting asynthetic resin, particularly a vinyl resin, having controlled shape,particle size (diameter) and texture to an appropriate oxidationtreatment and then to an appropriate carbonization process, provides anegative electrode material for non-aqueous electrolyte secondarybatteries exhibiting high output performances, high charge-dischargecapacities and high reliability, whereby the present invention has beenarrived at.

The negative electrode material for non-aqueous electrolyte secondarybatteries according to the present invention is based on the abovefindings and, more specifically, comprises: a carbon material having asphericity of at least 0.8, and exhibiting an average (002) interlayerspacing d₀₀₂ of 0.365-0.400 nm, a crystallite size in a c-axis directionLc₍₀₀₂₎ of 1.0-3.0 nm, as measured by X-ray diffractometry, ahydrogen-to-carbon atomic ratio (H/C) of at most 0.1 as measured byelementary analysis, and an average particle size Dv₅₀ of 1-20 μm.

The present invention further provides a process for producing theabove-mentioned negative electrode material, an electrode fornon-aqueous electrolyte secondary batteries formed by shaping thenegative electrode material together with a binder, and a non-aqueouselectrolyte secondary battery including the electrode as a negativeelectrode.

BEST MODE FOR PRACTICING THE INVENTION

The negative electrode material for non-aqueous electrolyte secondarybatteries is a spherical non-graphitizable carbon. The degree of thesphericity thereof is represented by a (true) sphericity (particularly,a circularity C obtainable according to a method described hereinafter),and a value closer to 1 means closer to the true sphericity. Thenegative electrode material of the present invention has a sphericity ofat least 0.8, preferably 0.90 or higher, further preferably 0.95 orhigher.

The texture of the non-graphitizable carbon used in the presentinvention is characterized by an average (002) place spacing d₀₀₂ of atleast 0.365 nm and at most 0.400 nm, and a crystallite size in a c-axisdirection Lc₍₀₀₂₎ of 1.0-3.0 nm, as measured by X-ray diffractometry. Asmaller average interlayer spacing represents a crystal structure whichis a characteristic of graphitizable carbon having a turbostratictexture or a graphitic material obtained by treatment thereof at hightemperatures. Such a graphitizable carbon material having a turbostratictexture or a graphitic material causes expansion and constriction at thetime of doping and dedoping reactions of lithium, and are not preferredin view of problems in durability. Too large an average interlayerspacing represents an insufficient degree of carbonization and causes anon-favorable increase of irreversible capacity which is a valueobtained by subtracting a dedoping capacity from a doping capacity,respectively of lithium. The average interlayer spacing d₀₀₂ ispreferably 0.365-0.400 nm, further preferably 0.370 -0.390 nm. As forthe crystalline texture of carbon materials, it is difficult todifferentiate a graphitizable carbon and a non-graphitizable carbon onlyby such an average interlayer spacing. Even a graphitizable carbon canexhibit an interlayer spacing larger than 0.350 nm if it has beancarbonized at a heat treatment temperature of 1000° C. or below, andeven a non-graphitizable carbon can exhibit an interlayer spacing below0.365 nm if it has been treated at a temperature of 2000° C. or higher.A non-graphitizable carbon having a smaller interlayer spacing is ratherrich in closed pores and is provided with remarkably decreased dopableand dedopable amounts of lithium (charge-discharge capacity). Also fromthis viewpoint, the average interlayer spacing is preferably 0.365-0.400nm, further preferably 0.370-0.390 nm.

A parameter for characterizing the texture of a non-graphitizable carbonother than the average interlayer spacing is a crystallite size inc-axis direction Lc₍₀₀₂₎. For a non-graphitizable carbon, thecrystallite size Lc₍₀₀₂₎ tends to increase as the heat treatmenttemperature becomes higher, but does not exceed 10 nm even when heatedat 3000° C. unlike that of a graphitizable carbon. Similarly as asmaller average interlayer spacing, a crystallite size Lc₍₀₀₂₎ in excessof 3 nm is accompanied with an undesirable increase of closed pores,leading to a lowering in doping-dedoping capacities. On the other hand,a crystallite size smaller than 1.0 nm represents an undesirablyinsufficient formation of carbon skeleton, and also is accompanied witha problem that an accurate measurement becomes difficult because ofremarkably weaker intensity of (002) diffraction lines. Accordingly, thecrystallite size Lc₍₀₀₂₎ is preferably 1.0-3.0 nm, further preferably1.0-2.5 nm, particularly preferably 1.0-2 nm.

A hydrogen-to-carbon atomic ratio (H/C) as measured by elementaryanalysis provides a good index of carbonization degree of a carbonmaterial. A lower carbonization degree is accompanied with an increaseof undesirable irreversible capacity due to abundant presence offunctional groups reacting with lithium. The hydrogen-to-carbon atomicratio (H/C) is preferably at most 0.1, further preferably at most 0.05,particularly preferably 0.02 or below.

In order to improve the output characteristic, it is important to makethinner the active substance layer in the electrode, and for thispurpose, a smaller average particle size is important. However, toosmall an average particle size leads to an undesirably increased amountof fine powder, which adversely affects the safety. Further, too small aparticle size requires a larger amount of binder, which results in anundesirable increase of electrode resistance. On the other hand, alarger average particle size results in an undesirable increase indiffusion free path of lithium, by which quick charge-discharge becomesdifficult. The average particle size in terms of Dv₅₀ (i.e., a particlesize giving a cumulative volume of 50%) is preferably 1-20 μm, furtherpreferably 4-15 μm, particularly preferably 4-10 μm.

In order to improve the output characteristic, it is important to makethinner the active substance layer in the electrode, and for thispurpose, a smaller maximum particle size is important. The maximumparticle size is preferably at most 50 μm, further preferably at most 40μm. A smaller maximum particle size leads to a smaller average particlesize. Thus, it is preferred for the negative electrode material of thepresent invention to have a uniform particle size distribution inaddition to the above-mentioned average particle size, whereby itbecomes possible to form a uniformly thin active substance layer. Morespecifically, a preferable uniformity of particle size of the negativeelectrode material according to the present invention is represented bya particle size disperse factor D₄/D₁ defined as a ratio of aweight-average particle size D₄(=Σ(nD⁴)/Σ(nD³)) to a length-averageparticle size D₁(=ΣnD/Σn) of at most 3.0, preferably at most 2.0,particularly preferably at most 1.5.

In order to suppress the decomposition of an electrolyte solution, it ispreferred to provide a smaller specific surface area. However, theaverage particle size and the specific surface area are in arelationship of mutually inverse proportion, and a smaller averageparticle size tends to result in a larger specific surface area. In thepresent invention, by providing the negative electrode material with ashape of sphere which can minimize the specific surface area per volumeand also by optimization of the texture and controlling the surfacestructure of the synthetic resin as a carbon precursor, it has becomepossible to provide a remarkably smaller specific surface area even at aparticle size smaller than that of a negative electrode materialobtained through pulverization. From this viewpoint, it is preferred toprovide as small a product of specific surface area S(m²/g) and averageparticle size Dv₅₀ as possible. On the other hand, too small a productof specific surface area S(m²/g) and average particle size Dv50 leads toundesirable obstruction of the doping-dedoping reactions of lithium. Theproduct of S(m²/g) and Dv₅₀ is preferably 3-40, further preferably 3-30.

In order to provide a negative electrode carbon material with arelatively small surface area, it is also preferred to incorporate ca.0.5 -5 wt. % of nitrogen. The incorporation of an excessive amount ofnitrogen leads to an undesirable increase of reaction with lithium.

It is preferred for the negative electrode carbon material of thepresent invention to have a surface structure coated with a siliconcompound or a surface texture with enhanced carbonization, for thepurpose of suppressing the increase of irreversible capacity due tosurface oxidation in the air. The coating rate of the silicon compoundis preferably 0.1-10 wt. %, further preferably 0.5-5 wt. %, in terms ofan amount of silicon oxide remaining after combustion of the sphericalcarbon material in an atmosphere of oxidizing gas such air (with respectto the carbon material). On the other hand, in order to provide thesurface texture with an enhanced carbonization degree, a highercarbonization temperature is preferred. An exothermic peak temperaturein air according to differential thermal analysis gives a good index ofthe surface oxidizability in air of a negative electrode carbonmaterial. A higher exothermic peak temperature is preferred as itrepresents a difficulty of surface oxidation, but too high an exothermicpeak temperature leads to an undesirable lowering in dedoping capacity.The exothermic peak temperature is preferably 600-700° C., furtherpreferably 610-690° C., particularly preferably 620-680° C.

A bulk specific gravity varies depending on the shape and particle sizedistribution of a carbon material and the micro texture of carbon and istherefore further useful for expressing the characteristic of thenegative electrode material according to the present invention. Too lowa bulk specific gravity undesirably lowers the density of the electrodeactive substance layer to result in a lower energy density per batteryvolume. On the other hand, too high a bulk specific gravity undesirablydecreases the void to lower the mobility of lithium ions in theelectrolyte solution, thus making difficult a quick discharge. In casewhere too high a bulk specific gravity is caused not by a particle shapebut by a high true density of the particles, it is accompanied withundesirable expansion and constriction of crystallites of the particles,thus resulting in lower repetition characteristics. The bulk specificgravity is preferably at least 0.40 and below 0.60, further preferably0.50-0.58.

The spherical carbon material of the present invention is obtainable bysubjecting a spherical synthetic resin to an oxidation treatment in anoxidizing gas atmosphere at a temperature of 150-400° C. to form athermally infusible spherical carbon precursor, and then carbonizing thecarbon precursor in a non-oxidizing gas atmosphere. Too low aheat-treatment temperature in the non-oxidizing gas atmosphereundesirably results in an increase of irreversible capacity of theresultant negative electrode material and also an increase ofirreversible capacity with time when left standing in an oxidizing gasatmosphere. On the other hand, too high a heat treatment temperatureundesirably lowers the dedoping capacity. The heat-treatment temperatureis preferably 1050-1500° C., further preferably 1100-1500° C.,particularly preferably 1200-1400° C.

As a spherical synthetic resin preferably usable in the presentinvention, it is possible to use a thermosetting resin, such as phenolicresin or furan resin, but it is particularly preferred to use a carbonprecursor obtained by applying an infusibilization treatment to asphered thermoplastic resin, particularly a vinyl resin obtained byradical polymerization. The sphering of a thermoplastic resin can beeffected, e.g., by dispersion of a molten resin into a gas or hot water,but may preferably be effected by suspension polymerization in anaqueous dispersion medium.

A spherical vinyl resin particularly preferably usable in the presentinvention may, for example, be obtained in the following manner. Thus, amonomer mixture comprising a radically polymerizable vinyl monomer and apolymerization initiator is added into an aqueous dispersion mediumcontaining a dispersion stabilizer and suspended under stirring-mixingto form fine liquid droplets, and the system is elevated in temperatureto proceed with radical polymerization, thereby forming a vinyl monomerin a true spherical form.

The vinyl monomer can be any vinyl monomer capable of forming a vinylresin which in turn can provide a carbon precursor through oxidation,whereas in order to provide a crosslinked vinyl resin giving anincreased carbonization yield, a vinyl monomer mixture containing acrosslinking agent is preferably used. Further, from the viewpoints ofproviding a high carbonization yield from the resultant spherical vinylresin and also a spherical carbon exhibiting preferable batteryperformance, it is particularly preferred to use a starting monomermixture comprising 10-80 wt. % of a styrene monomer, 10-90 wt. % of anacrylonitrile monomer, and a crosslinking agent in a proportion of atleast 15 wt. % of the styrene monomer.

The styrene monomer includes, in addition to styrene; styrene derivativeobtainable by replacing the vinyl group-forming hydrogen or phenylgroup-forming hydrogen of styrene with a substituent, and compoundsobtainable by bonding the vinyl group to a heterocyclic or polycycliccompound instead of the phenyl group of styrene. More specifically,representative examples thereof may include; α- or β-methylstyrene, α-or β-ethylstyrene, methoxystyrene, phenylstyrene, and chlorostyrene; o-,m- or p-methylstyrene, ethylstyrene, methylsilylstyrene, hydroxystyrene,cyanostyrene, nitrostyrene, aminostyrene, carboxystyrene andsulfoxystyrene; sodium styrenesulfonate; vinylpyridine, vinylthiophene,vinylpyrrolidone, vinylnaphthalene, vinylanthracene, and vinylphenyl.

The acrylonitrile monomer includes acrylonitrile and methocrylonitrile,of which acrylonitrile is preferred from the economical viewpoint.

It is preferred that the styrene monomer is contained at 10-80 wt. %,particularly 20-70 wt. %, in the monomer mixture. If the styrene monomeris below 10 wt. %, the content of the acrylonitrile monomer which isrelatively water-soluble is increased, so that the formation of monomerdroplets with a good sphericity is liable to be difficult duringsuspension polymerization. A styrene monomer content exceeding 80 wt. %is not preferred because it naturally reduces the contents of theacrylonitrile monomer and the crosslinking agent.

On the other hand, it is preferred that the acrylonitrile monomer iscontained at 10-90 wt. % (more exactly at most 88.5 wt. % in view of theminimum content of the crosslinking agent), more preferably 20-80 wt. %,particularly preferably 30-70 wt. %, in the monomer mixture. Theacrylonitrile monomer advantageously functions to increase thecarbonization yield of the resultant vinyl resin and to decrease thespecific surface area of the resultant spherical carbon material,thereby suppressing the decomposition of the electrolyte at the carbonsurface when used as the negative electrode material of a non-aqueouselectrolyte secondary battery. An acrylonitrile monomer below 10 wt. %leads to insufficiency of the above effect, and in excess of 90 wt. %,the resultant spherical vinyl resin is provided with an undesirably lowsphericity.

It is preferred that the monomer mixture contains a crosslinking agentin a proportion of at least 15 wt. %, particularly 20 wt. % or more ofthe styrene monomer, with the proviso that the styrene monomer or theacrylonitrile monomer will not subside 10 wt. % that is the lower limitof each monomer in the monomer mixture. In case where the crosslinkingagent is less than 15 wt. % of the styrene monomer, the spherical vinylresin is liable to decompose or melt during the oxidation treatment(infusibilization treatment) so that the oxidation treatment becomesdifficult.

The crosslinking agent may be selected from the group consisting of;divinylbenzene, divinylpyridine, divinyltoluene, divinylnaphthalene,diallyl phthalate, ethylene glycol diacrylate, ethylene glycoldimethylate, divinylxylene, divinylethylbenzene, divinylsulfone;polyvinyl or poly allyl ethers of glycols or glycerols, pentaerythritol,mono-or di-thio derivatives of glycols, and resorcinol; divinyl ketone,divinyl sulfide, allyl acrylate, diallyl maleate, diallyl fumarate,diallyl succinate, diallyl carbonate, diallyl malonate, diallyl oxalate,diallyl adipate, diallyl sebacate, triallyl tricarballylate, triallylaconitate, triallyl citrate, triallyl phosphate,N,N′-methylenediacrylamide,1,2-di(α-methylmethylenesulfonamido)ethylene, trivinylbenzene,trivinylnaphthalene, polyvinylanthracene and trivinylcyclohexane.Particularly preferred examples of the crosslinking agent may includepolyvinylaromatic hydrocarbons (e.g., divinylbenzene), glycoltrimethacrylates (e.g., ethylene glycol dimethacrylate) and polyvinylhydrocarbons (e.g., trivinylcyclohexane). The most preferred one isdivinylbenzene because of its thermal decomposition characteristic.

The polymerization initiator is not particularly restricted but may beany one generally used in this field, whereas an oil-solublepolymerization initiator soluble in the polymerizable monomer ispreferred. Examples of the polymerization initiator may include dialkylperoxides, diacyl peroxides, peroxy esters, peroxydicarbonate and azocompounds. More specifically enumerated are; dialkyl peroxides, such asmethyl ethyl peroxide, di-t-butyl peroxide and dicumyl peroxide; diacylperoxides, such as isobutyl peroxide, benzoyl peroxide,2,4-dichlorobenzoyl peroxide, and 3,5,5-trimethylhexanoyl peroxide;peroxy esters, such as t-butyl peroxypivalate, t-hexyl peroxy pivalate,t-butyl peroxy-neodecanoate, t-hexyl peroxyneodecanoate,1-cyclohexyl-1-methylethyl peroxyneodecanoate, 1,1,3,3-tetramethylbutylperoxyneodecanoate, cumyl peroxyneodecanoate, and (α,α-bis-neodecanoylperoxy)diisopropylbenzene; peroxydicarbonates, such asbis(4-t-butylcyclohexyl)peroxydicarbonate, di-n-propylperoxydicarbonate, diisopropyl peroxydicarbonate,di(2-ethylethyl)peroxydicarbonate, dimethoxybutyl peroxydicarbonate anddi(3-methyl-3-methoxybutyl)peroxydicarbonate; and azo compounds, such as2,2′-azobisisobutyronitrile,2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile),2,2′-azobis(2,4-dimethylvaleronitrile) and1,1′-azobis(1-cyclohexanecarbonitrile).

The dispersion stabilizer used in the suspension polymerization is notparticularly restricted, whereas silica is preferably used as astabilizer since if silica (colloidal silica) is used as the stabilizerand the resultant true-spherical synthetic resin carrying the silica onits surface is carbonized to form a stable film on the resultant carbon,surface oxidation liable to occur during standing of the carbon materialcan be suppressed thereby. A dispersion liquid medium containing such adispersion stabilizer may be prepared ordinarily by using deionizedwater together with an auxiliary stabilizer optionally added thereto. Inthe case of using colloidal silica as the dispersion stabilizer, it ispreferred to effect the polymerization in an acidic medium. In the caseof using colloidal silica, it is preferred to use a condensation productas an auxiliary stabilizer, and preferred examples of the condensationproduct may include condensates of diethanol amine and aliphaticdicarboxylic acids and condensates of aliphatic dicarboxylic acids, ofwhich a condensate of diethanol amine adipate and itaconic acid isparticularly preferred.

As the dispersion stabilizer, it is also possible to use methylcellulose. In the case of using methyl cellulose, a stable film on thecarbon surface is not formed unlike the case of using colloidal silica,but a favorable output characteristic is obtained because the absence ofsuch a film on the carbon surface allows easy going into and out of thenegative electrode carbon material of the active substance.

Further addition of an inorganic salt, such as sodium chloride, sodiumsulfate or sodium nitrite, as an auxiliary stabilizer allows theproduction of a resin having a more uniform particle shape.

The particle size of the spherical synthetic resin obtained throughsuspension polymerization can be controlled based on the size of theliquid droplets. In order to provide a preferred average particle sizeof 1-20 μm for the negative electrode material of the present invention,the synthetic resin may preferably have a particle size of 5-30 μm,further preferably 5-20 μm.

The thus-obtained spherical synthetic resin can be converted into aspherical carbon precursor through an oxidation (infusibilization)treatment thereof for developing a crosslinked structure. The oxidationtreatment may preferably be performed at a temperature of 150° C. to400° C. As the oxidizing agent, it is possible to use an oxidizing gas,such as O₂, O₃, SO₃, NO₂, a mixture gas obtained by diluting these withair, nitrogen, etc., or an oxidizing gas such as air, or an oxidizingliquid, such as sulfuric acid, nitric acid or aqueous hydrogen peroxide.

By heat-treating the spherical carbon precursor at a temperature of1000-1500° C., it is possible to produce a spherical carbon materialsuitable as a negative electrode material for non-aqueous electrolytesecondary batteries according to the present invention. Theheat-treatment temperature is preferably 1050-1500° C., furtherpreferably 1100-1500° C., particularly preferably 1200-1400° C.

The spherical carbon material of the present invention obtained in theabove-described manner may, for example, be used for production ofelectrodes, as it is or together with an electroconductive aidcomprising, e.g., electroconductive carbon black, such as acetyleneblack or furnace black in an amount of 1-10 wt. % thereof, incombination with a binder and an appropriate amount of solvent addedthereto, followed by kneading to form a pasty electrode-formingcomposition, which is then applied onto an electroconductive substratecomprising, e.g., a circular or rectangular metal plate, dried andpress-formed into a 10 to 200 μm-thick layer. The binder is notparticularly restricted if it is not reactable with an electrolyticsolution and may comprise polyvinylide fluoride, polytetrafluorethylene,styrene butadiene rubber (SBR), etc. In the case of polyvinylidenefluoride, a solution thereof in a polar solvent, such asN-methylpyrolidone (NMP), may preferably be used, whereas it is alsopossible to use an aqueous emulsion of SBR, etc. The binder maypreferably be added in an amount of 0.5-10 wt. parts per 100 wt. partsof the spherical carbon material according to the present invention. Toolarge an addition amount of the binder is not preferred because itresults in an increase in electrical resistance of the resultantelectrode leading to an increased inner resistance of the battery andlower battery performances. On the other hand, too small an additionamount of the binder results in insufficient bonding of the sphericalcarbon material particles with each other and with the electroconductivesubstrate. The spherical carbon material of the present invention maypreferably be used as an active substance of a negative electrode of anon-aqueous electrolyte secondary battery, particularly as a negativeelectrode active substance for a lithium secondary battery, by takingadvantage of excellent doping characteristic thereof. The coating amountof the active substance is preferably as small as possible, so as toprovide a larger output, and may preferably be at most 60 g/m², furtherpreferably 50 g/m² or lower.

In the case of forming a negative electrode of a non-aqueous electrolytesecondary battery, other components of the battery, such as a positiveelectrode material, a separator and an electrolytic solution, are notparticularly restricted, and various materials conventionally used in orproposed to be used for non-aqueous electrolyte secondary batteries canbe used.

For example, the positive electrode material may preferably comprise acomplex metal chalcogenide, such as LiCoO₂, LiNiO₂, LiMnO₂, or LiMn₂O₄,and may be formed together with an appropriate binder and anelectroconductivity-imparting carbon material into a layer on anelectroconductive substrate.

A non-aqueous solvent-type electrolytic solution used in combinationwith such a positive electrode and a negative electrode may generally beformed by dissolving an electrolyte in a non-aqueous solvent, it ispossible to use one or two or more species in combination of organicsolvents, such as propylene carbonate, ethylene carbonate, dimethylcarbonate, diethyl carbonate, dimethoxyethane, diethoxyethane,γ-butyrolactone, tetrahydrofuran, 2-methyl-tetrahydrofuran, sulfolaneand 1,3-dioxolane. On the other hand, as the electrolyte, it is possibleto use LiClO₄, LiPF₆, LiBF₄, LiCF₃SO₃, LiAsF₆, LiCl, LiBr, LiB(C₆H₅)₄,LiN(SO₃CF₃)₂, etc. A secondary battery may generally be formed byoppositely disposing a positive electrode layer and a negative electrodelayer prepared in the above-described manner optionally by the medium ofa liquid-permeating separator comprising non-woven cloth, another porousmaterial, etc., and immersing them in an electrolytic solution.

It is also possible to use a solid electrolyte comprising a polymer gelimpregnated with an electrolytic solution instead of such a separator.

EXAMPLES

Hereinbelow, the present invention will be described more specificallywith reference to Examples. Physical property values described in thespecification including the following Examples are based on valuesobtained according to the following method.

(1) Measurement of Particle Size Distribution:

Three drops of dispersing agent (a cationic surfactant: “SN DISPERSANT7347-C”, made by San Nopco K.K.) was added to ca. 0.1 g of a sample towet the sample with the dispersing agent. Then, 30 ml of deionized waterwas added thereto, and the mixture was subjected to dispersion by meansof an ultrasonic washing machine for ca. 2 min, and to measurement of aparticle size dispersion in a particle size range of 0.5-3000 μm byusing a particle size distribution meter (“SALD-3000J”, made by ShimadzuCorporation).

(2) Average Particle Size Dv₅₀(μm):

From the particle size distribution determined in the above section (1),a particle size giving a cumulative volume of 50% was taken as anaverage particle size Dv₅₀(μm).

(3) Particle Size Distribution Factor D₄/D₁:

Determined as a ratio D₄/D₁ between a weight-average particle sizeD₄(=Σ(nD⁴)/Σ(nD³)) and a length-average particle size D₁=(ΣnD/Σn)(wherein D denotes a particle size of individual particles and n denotesthe number of particles having the particle size) respectively obtainedfrom the particle size distribution determined in the above section (1).

(4) Sphericity:

Carbon material particles were embedded in epoxy resin and, after beingpolished, the sample was observed through an optical microscope, then,30 particles that had a particle size in a range of the average particlesize Dv₅₀±50% and were free from overlapping or contact with otherparticles, were selected and subjected to a planar image analysis of theparticles by means of a high-performance image analysis system(“IP-500PC”, made by Asahi Engineering K.K.) to determine a sphericityin terms of an average value of circularities C calculated according tothe following formula;

C=4πS/l ²,

wherein l denotes a circumferential length, and S denotes an area.

(5) Bulk Specific Gravity:

A bulk specific gravity was measured according to JIS K-6721: 1977. Morespecifically, ca. 120 mg of a sufficiently stirred sample was placed ina funnel bottomed with a dumper of a bulk specific gravity meter (madeby K.K. Kuramochi Kagaku Kiki Seisakusho), and then the dumper waswithdrawn to drop the sample into a receiver vessel (100±0.5 ml). Anamount of the sample rising above the receiver vessel was slitted off bya glass rod, and the vessel containing the sample was accurately weighedat an accuracy of 0.1 g. The bulk specific gravity was calculated downto 2 digits below a decimal point according to the following formula.The measurement was repeated 3 times to take an average value thereof.

Bulk Specific Gravity

=(the weight of the vessel containing the sample (g)−the weight of thevessel alone (g))/the inner volume of the vessel (ml).

(6) Average Interlayer Spacing d₀₀₂ of a Carbon Material:

A powdery sample of a carbon material was packed in a sample holder andirradiated with monochromatic CuK_(α) ray through a graphitemonochrometer to obtain an X-ray diffraction pattern. The peak positionof the diffraction pattern was determined by the center of gravitymethod (i.e., a method wherein the position of gravity center ofdiffraction lines is obtained to determine a peak position as a 2θ-value corresponding to the gravity center) and calibrated by thediffraction peak of (111) plane of high-purity silicon powder as thestandard substance. The d₀₀₂ value was calculated by the Bragg's formulawith the wavelength of the CuK_(α) ray as 0.15418 nm. Further, bysubtracting a half-value width of (111) diffraction lines of the siliconpowder from a half-value width obtained from the integration of the(002) diffraction lines to obtain a β value, from which a thicknessLc₍₀₀₂₎ of crystallites in the C-axis direction was calculated by theScherrer's formula.

d ₀₀₂=λ/2·sin θ  (Bragg's formula)

Lc ₍₀₀₂₎=κλ/β_(1/2)·cos θ  (Scherrer's equation)

(7) Hydrogen/Carbon (H/C) Atomic Ratio

A sample was subjected to elementary analysis by using a CHN analyzer,and a hydrogen/carbon (H/C) atomic ratio was calculated from the weightproportions of hydrogen and carbon in the sample.

(8) Specific Surface Area:

An approximate equation: v_(m)=1/(v·(1−x)) derived from the BET equationwas used to obtain v_(m) at the liquid nitrogen temperature according tothe BET single point method (at a relative pressure x(=0.3)) usingnitrogen adsorption, and a specific surface area of the sample wascalculated based on the following equation:

specific surface area=4.35×v _(m)(m ² /g),

wherein v_(m) denotes an amount of adsorption (cm³/g) required to form amono-molecular layer, v denotes an actually measured amount ofadsorption (cm³/g), and x denotes a relative pressure.

More specifically, an amount of adsorbed nitrogen on a carbon materialat the liquid nitrogen temperature was measured in the following mannerby using “Flow Sorb II2300” made by Micromeritics Instrument Corp.

A sample carbon material pulverized to a particle diameter of ca. 5-50μm was packed in a sample tube, and the sample tube was cooled to −196°C. while flowing helium gas containing nitrogen at a concentration of 30mol %, thereby to cause the carbon material to adsorb nitrogen. Then,the sample tube was restored to room temperature to measure the amountof nitrogen desorbed from the sample by a thermal conductivity-typedetector, thereby to obtain the adsorbed amount of the gas v.

(9) Exothermic Peak Temperature According to Differential ThermalAnalysis:

Carbon sample powder was weighed at 2.0 mg on a platinum-made pan andset in a differential thermal analyzer (“DTG-50”, made by ShimadzuCorporation), and while flowing dry air (having a dew point of −50° C.or below) at a rate of 100 ml/min, the carbon sample was held at 200° C.for 1 hour and then heated at a temperature-raising rate of 10° C./minto obtain an exothermic curve due to oxidation of the carbon material,from which a temperature showing a maxim exothermic quantity was takenas an exothermic peak temperature.

Examples and Comparative Examples are described below, wherein “%”representing a proportion of component means a wt. % unless otherwisenoted specifically.

Example 1

Into 5176 kg of water, 32 g of colloidal silica (160 g of silicadispersion liquid having a solid content of 20 wt. %), 3.96 g ofdiethanolamine-adipic acid condensation product (acid value=75 mg KOH/g)(7.92 g as a 50 wt. % liquid) and 0.99 g of sodium nitrite weresuccessively added to prepare an aqueous dispersion medium, to whichhydrochloric acid was added to provide a pH of ca. 3.5, followed by 10minutes of a dispersion treatment by means of a homogenizer at 8000 rpm.On the other hand, 890 g of acrylonitrile (AN), 823 g of styrene (St),266 g of divinylbenzene (DVB) and 10.69 g of2,2′-azobis-2,4-dimethylvaleronitrile were blended to prepare a monomermixture (corresponding to a monomer mixture obtained by blending amixture A of St/DVB=76%/24% with AN at a ratio of mixture A/AN=55/45%,for convenience). The monomer mixture and the aqueous dispersion mediumwere stirred for 2 minutes at 3200 rpm by a homogenizer to form minutedroplets of the monomer mixture. The aqueous dispersion mediumcontaining the minute droplets of the polymerizable mixture was chargedin a polymerization vessel (10 L) equipped with a stirrer, and subjectedto reaction for 1 hour at 55° C. on a warming bath. Into the system, adilution of 1.7 g of silane coupling agent with 42.8 g of acidic water(pH 3.5) was charged and, 30 minutes thereafter, 27 g of 1% dilutehydrochloric acid was added, followed by further 20 hours of reaction at55° C. The resultant polymerization product was filtered out from theaqueous phase, dried and disintegrated by a jet mill to obtain atrue-spherical vinyl resin having an average particle size (Dv₅₀) of 17μm.

60 g of the thus-obtained true-spherical vinyl resin was charged in aquartz-made vertical annular furnace equipped with a dispersion plateand caused to form a fluidized bed thereof, while blowing air upwards,followed by 1 hour of oxidation at 280° C. to form a spherical carbonprecursor. The carbon precursor was found to have an oxygen content of15 wt. % as a result of elementary analysis. The spherical carbonprecursor was heat-treated for 1 hour in nitrogen to form apreliminarily calcined carbon, which was then placed in a horizontaltubular furnace, heated to 1200° C. in a nitrogen atmosphere andretained for 1 hour for main calcination, followed by cooling to form aspherical carbon material having an average particle size of 10 μm.

Some representative features of the-thus obtained carbon material areinclusively shown in Table 1 appearing hereinafter together with thoseof carbon materials obtained in the following Examples and ComparativeExamples.

Example 2

A spherical carbon material was prepared in the same manner as inExample 1 except for changing the main calcination temperature from1200° C. for 1 hour to 1300° C. for 1 hour.

Example 3

A spherical carbon material was prepared in the same manner as inExample 2 except for changing the temperature for oxidation of thespherical synthetic resin from 280° C. for 1 hour to 260° C. for 1 hourto change the oxygen content of the spherical carbon precursor from 15wt. % to 10 wt. %.

Example 4

A spherical carbon material was prepared in the same manner as inExample 2 except for changing the composition of the monomer mixture toAN 1800 g, St 77 g, DVB 103 g, and 2,2′-azobis-2,4-dimethylvaleronitrile10.69 g (mixture A: St/DVB=43%/57%; monomer mixture: mixtureA/AN=9%/91%).

Example 5

A spherical carbon material was prepared in the same manner as inExample 2 except for changing the composition of the monomer mixture toAN 1380 g, St 403 g, DVB 177 g, and2,2′-azobis-2,4-dimethylvaleronitrile 10.69 g (mixture A:St/DVB=70%/30%; monomer mixture: mixture A/AN=30%/70%).

Example 6

A spherical carbon material was prepared in the same manner as inExample 2 except for changing the composition of the monomer mixture toAN 590 g, St 977 g, DVB 413 g, and 2,2′-azobis-2,4-dimethylvaleronitrile10.69 g (mixture A: St/DVB=70%/30%; monomer mixture: mixtureA/AN=70%/30%), and changing the main calcination temperature from 1300°C. for 1 hour to 1350° C. for 1 hour.

Example 7

A spherical carbon material was prepared in the same manner as inExample 2 except for changing the composition of the monomer mixture toSt 1194 g, DVB 781 g, and 2,2′-azobis-2,4-dimethylvaleronitrile 10.69 g(mixture A: St/DVB=60%/40%; monomer mixture: mixture A/AN=100%/0%)

Example 8

A spherical carbon material was prepared in the same manner as inExample 2 except for omitting the steps of adding the dilution of is 1.7g of silane coupling agent with 42.8 g of acidic water (pH 3.5) and, 30minutes there after, adding 27 g of 1% hydrochloric acid, and removingthe colloidal silica from the polymerization product at time of thefiltration, followed by drying and disintegration to obtain a truespherical vinyl resin having an average particle size of 17 μm.

Example 9

A spherical carbon material was prepared in the same manner as inExample 8 except for changing the main calcination temperature from1300° C. for 1 hour to 1350° C. for 1 hour.

Example 10

An aqueous dispersion medium comprising 3750 kg of water, 1525 g of 1.44wt. %-methyl cellulose aqueous solution and 0.99 g of sodium nitrite wasprepared. On the other hand, a monomer mixture comprising AN 675 g, St375 g, DVB 440 g and 10.69 g of 2,2′-azobis-2,4-dimethylvaleronitrilewas prepared. The monomer mixture and the aqueous dispersion medium werestirred for 20 minutes at 3000 rpm by a homogenizer to form minutedroplets of the monomer mixture. The aqueous dispersion mediumcontaining the minute droplets of the polymerizable mixture was chargedin a polymerization vessel (10 L) equipped with a stirrer, and subjectedto 20 hours of polymerization at 55° C. on a warming bath. The resultantpolymerization product was filtered out from the aqueous phase, driedand disintegrated by a jet mill to obtain a true-spherical vinyl resinhaving an average particle size of 38 μm.

The thus-obtained true-spherical synthetic resin was subjected to thesame treatments as in Example 1 except for changing the main calcinationtemperature from 1200° C. for 1 hour to 1350° C. for 1 hour to obtain atrue-spherical carbon material.

Comparative Example 1

The true spherical synthetic resin obtained in Example 1 was subjectedto the preliminary calcination while omitting the oxidation treatment,whereby the resin was caused to melt and foam, thus failing to providean objective spherical carbon material.

Comparative Example 2

A spherical carbon material was prepared in the same manner as inExample 1 except for changing the main calcination temperature from1200° C. for 1 hour to 900° C. for 1 hour.

Comparative Example 3

A true spherical vinyl resin was prepared in the same manner as inExample 1 except for changing the composition of the monomer mixture toSt 1750 g, DVB 200 g, and 2,2′-azobis-2,4-dimethylvaleronitrile 10.69 g(mixture A: St/DVB=89%/10%; monomer mixture: mixture A/AN=100%/0%). Thevinyl resin was subjected to the oxidation treatment in the same manneras in Example 1, whereby the synthetic resin was melted to fail inproviding a spherical carbon precursor.

Comparative Example 4

68 kg petroleum pitch having a softening temperature of 210° C., aquinoline-insoluble content of 1 wt. % and an H/C atomic ratio of 0.63,and 32 kg naphthalene, were placed in a 300 liter-pressure-resistantvessel equipped with stirring blades, melt-mixed under heating at 190°C. and, after being cooled to 80-90° C., extruded to form a ca. 500 μmdia.-string-shaped product. Then, the string-shaped product was brokenso as to provide a diameter-to-length ratio of ca. 1.5, and the brokenproduct was charged into an aqueous solution containing 0.53 wt. % ofpolyvinyl alcohol (saponification degree=88%) and heated to 93° C.,followed by stirring for dispersion and cooling to form a slurry ofpitch spheres. After removing a major part of water by filtration, thepitch spheres were subjected to extraction with ca. 6 times by weight ofn-hexane to remove the naphthalene in the pitch spheres. Thethus-obtained porous spherical pitch was heated to 260° C. in afluidized bed while passing heated air and held at 260° C. for 1 hour tobe oxidized into a thermally-infusible porous spherical oxidized pitch.The oxidized pitch was found to have. an oxygen content of 17 wt. %. Theoxidized pitch was then heated to 600° C. in a nitrogen gas atmosphere(normal pressure) and held at 600° C. for 1 hour for preliminary heatingto obtain a carbon precursor having a volatile matter content of at most2%. The carbon precursor was pulverized to form a powdery carbonprecursor having an average particle size of 10 μm, which was thencharged in a calcination furnace of a nitrogen gas atmosphere, heated to1200° C. and held at that temperature for 1 hour for main calcination,followed by cooling to obtain a powdery carbon material.

Comparative Example 5

The porous spherical pitch prepared in the same manner as in ComparativeExample 4 was heated to 160° C. in a fluidized bed while passing heatedair and held at 160° C. for 1 hour to form an porous spherical oxidizedpitch. The oxidized pitch was found to have an oxygen content of 2 wt.%. The oxidized pitch was then heated to 600° C. in a nitrogen gasatmosphere (normal pressure) and held at 600° C. for 1 hour to becrystallized into a carbon precursor having a volatile matter content ofat most 2%. The carbon precursor was pulverized to form a powdery carbonprecursor having an average particle size of 12 μm, which was thencharged in a calcination furnace, heated in a nitrogen stream to 1200°C. and held at 1200° C. for 1 hour for main calcination followed bycooling to form a powdery carbon material having an average particlesize of 10 μm.

Comparative Example 6

Needle coke was pulverized to form a powdery carbon precursor having anaverage particle size of 12 μm. The powdery carbon precursor was thencharged in a calcination furnace, heated in a nitrogen stream to 1200°C. and held at 1200° C. for 1 hour for main calcination, followed bycooling to form a powdery carbon material having an average particlesize of 10 μm.

Comparative Example 7

True-spherical form phenolic resin having an average particle size of 17μm (“MARILIN”, made by Gun Ei Kagaku K.K.) was heated to 600° C. in anitrogen gas atmosphere (normal pressure) and held at 600° C. for 1 hourfor preliminary calcination to obtain a spherical carbon precursorhaving a volatile content of at most 2%. Then, the spherical carbonprecursor was charged in a calcination furnace, heated in a nitrogenstream to 1200° C. and held at 1200° C. for 1 hour for main calcination,followed by cooling to form a true-spherical carbon material.

Comparative Example 8

A true-spherical carbon material was prepared in the same manner as inComparative Example 7 except for changing the main calcinationconditions from 1200° C. for 1 hour to 1000° C. for 1 hour.

Comparative Example 9

A true-spherical carbon material was prepared in the same manner as inComparative Example 8 except for changing the main calcinationconditions from 1300° C. for 1 hour to 1080° C. for 1 hour.

(Doping/Dedoping Capacity for Active Substance)

Electrodes were formed by using the carbon materials obtained in theabove-described Examples and Comparative Examples, and the electrodeperformances and the preservability thereof were evaluated, through thefollowing steps (a)-(f).

(a) Preparation of an Electrode.

90 wt. parts of a carbon material as described above and 10 wt. parts ofpolyvinylidene fluoride (“KF#1100” made by Kureha Chemical Industry Co.,Ltd.) were formed together with NMP into a paste composition P, whichwas then evenly applied onto a copper foil. After being dried, thecomposition was peeled from the copper foil and stamped into a 15mm-dia. disk. The amount of the carbon material in an electrode wasadjusted to ca. 20 mg.

(b) Preparation of a Test Cell.

The carbon material of the present invention is generally suited forconstituting a negative electrode of a non-aqueous electrolyte secondarybattery, but the above-prepared electrode was used to form a lithiumsecondary battery together with a counter electrode comprising lithiummetal showing stable properties so as to accurately evaluate thedischarge capacity (de-doping capacity) and irreversible capacity(non-de-doping capacity) of cell active substance without being affectedby a fluctuation in performances of the counter electrode.

More specifically, the above-prepared 15 mm-dia. disk formed from thecarbon material of each of the about Examples and Comparative Exampleswas press-bonded to a 17 mm-dia. disk-shaped net of stainless steelwhich had been spot-welded in advance to an inner lid of a coin-shapedcell can of 2016-size (i.e., 20 mm in diameter and 1.6 mm in thickness),to from an electrode.

The preparation of a lithium electrode was performed in a glove box ofan Ar atmosphere. A 17 mm-dia. disk-shaped net of stainless steel wasspot-welded in advance to an outer lid of the 2016-size coin-shaped cellcan, and a 0.5 mm-thick thin plate of lithium metal stamped into a 15mm-dia. disk was press-bonded onto the disk of stainless steel net toprovide a counter electrode.

The thus-prepared pair of electrodes were disposed opposite to eachother with a 17 mm-dia. polypropylene-made porous membrane as aseparator and assembled together with an electrolyte liquid comprising amixture solvent of propylene carbonate and dimethoxyethane mixed in avolume ratio of 1:1 and LiClO₄ added thereto at a rate of 1 mol/liter inan Ar-glove box to form a 2016-size coin-shaped non-aqueous electrolytelithium secondary battery (cell).

(c) Measurement of Cell Capacity.

A lithium secondary battery of the above-described structure wassubjected to a charge-discharge test by using a charge-discharge tester(“TOSCAT”, made by Toyo System K.K.). The charging and discharging wereperformed according to the constant current-constant voltage method. The“charging” is caused as a discharging reaction of the test cell but thereaction is caused by insertion of lithium into a carbon material and istherefore described herein as “charging” for conveniences. On the otherhand, the “discharging” is caused as a charging reaction of the testcell but is described herein as “discharging” since it is caused byliberation of lithium from the carbon material. Under the constantcurrent-constant voltage conditions adopted herein, the charging wascontinued at a constant current density of 0.5 mA/cm² until the cellvoltage reached 0 V, and thereafter charging was continued bycontinuously changing the current value so as to keep a constant voltageof 0 V until the current value reached 20 μA. The electricity supple atthis time was divided by the weight of the carbon material in theelectrode to provide a charge capacity per unit weight of carbonmaterial (mAh/g) defined herein. After completion of the charging, thecell circuit was made open for 30 minutes, thereafter the dischargingwas effected. The discharging was performed at a constant currentdensity of 0.5 mA/cm² until-the cell voltage reached 1.5 V, and theelectricity discharged at this time was divided by the weight of thecarbon material in the electrode to provide a discharge capacity perunit weight of carbon material (mAh/g) defined herein. An irreversiblecapacity was calculated as the charge capacity—the discharge capacity.

Charge-discharge capacities and irreversible capacity for a sample weredetermined by averages of measured values for a number of measurement of3 (n=3) performed by using test cells prepared for a single sample.

(d) Quick Charge-Discharge Test.

By using a lithium secondary battery (cell) of the above-describedstructure, the carbon material was charged in the same manner as in thesection (c) above, and after completion of the charging under theconstant current density, the cell circuit was made open for 30 minutes.There after, discharging was performed at a constant current density of20 mA/cm², and the electricity discharged at this time was divided bythe electrode to provide a quick discharge capacity (mAh/cm²) definedherein.

(e) Test for Preservability of Electrode Material

An irreversible capacity lo immediately after preparation (0 day) and anirreversible capacity I₃₀ after 30 days of storage in air (having a dewpoint of −60° C.) at 25° C. of a negative electrode material weremeasured according to the method described in the section (c) above, anda percentage of atmospheric deterioration was determined according tothe following formula:

((I ₃₀ −I ₀)/I ₀)×100.

(f) Repetition Performance Test

90 wt. parts of carbon material prepared in each of the above Examplesand Comparative Example and 10 wt. parts of polyvinylidene fluoride(“KF# 1100” made by Kureha Chemical Industry Co., Ltd.) were formedtogether with NMP into a paste composition P, which was then evenlyapplied onto a copper foil. After being dried, the composition waspeeled from the copper foil and stamped into a 15 mm-dia. disk to form anegative electrode. The amount of the carbon material was adjusted toca. 14 mg.

94 wt. parts of lithium cobaltate (LiCoO₂), 3 wt. parts of carbon blackand 3 wt. parts of polyvinylidene flurried (“KF#1100”, made by KurehaChemical Industry Co., Ltd.) were formed together with NMP to form apaste composition, which was then evenly applied onto an aluminum foil.After being dried, the coating electrode was stamped into a 14 mm-dia.disk. The amount of lithium cobaltate in the positive electrode wasadjusted so as to provide 80% of the charging capacity of the negativeelectrode active substance as measured in the suction (c) above, whileassuming the capacity of lithium cobaltate to be 150 mAh/g.

The thus-prepared pair of electrodes were disposed opposite to eachother with a 17 mm-dia. polypropylene-made porous membrane as aseparator and assembled together with an electrolyte liquid comprising amixture solvent of propylene carbonate and dimethoxyethane mixed in avolume ratio of 1:1 and LiPF₆ added thereto at a rate of 1 mol/liter inan Ar-glove box to form a 2016-size coin-shaped non-aqueous electrolytelithium secondary battery (cell).

Under the constant current-constant voltage conditions adopted herein,the charging was continued at a constant current density of 3 mA/cm²until the cell voltage reached 4.2 V, and thereafter charging wascontinued by continuously changing the current value so as to keep aconstant voltage of 4.2 V until the current value reached 50 μA. Aftercompletion of the charging, the cell circuit was made open for 30minutes, thereafter the discharging was effected. The discharging wasperformed at a constant current density of 3 mA/cm² until the cellvoltage reached 2.75 V. The charging and discharging were repeated in 25cycles at 25° C., then the cell was warmed to 45° C., and thecharge-discharge operation was repeated in further 100 cycles, whereby adischarge capacity after the 100 cycles was divided by the dischargecapacity in the first cycle after the warming to provide a capacityretention rate (%).

The electrochemical performances of the carbon materials of Examples andComparative Examples measured in the manners described in the abovesections (a)-(f) were inclusively shown in Table 2 together with somerepresentative physical properties of the carbon materials.

TABLE 1 Silica Silica content Resin composition content in OxygenCalcination Carbonization in the carbon wt % the resin content temp.yield N-content material St DVB AN % % ° C. % Sphericity H/C wt % %Example 1 41.8 13.2 45 1.6 15 1200 65 0.98 0.02 2.0 2.5 Example 2 41.813.2 45 1.6 15 1300 61 0.99 <0.01 1.8 2.9 Example 3 41.8 13.2 45 1.6 101300 48 0.99 <0.01 1.6 3.5 Example 4 3.87 5.13 91 1.6 15 1300 45 0.99<0.01 3.8 2.3 Example 5 20.3 8.7 70 1.6 15 1300 62 0.99 <0.01 3.2 2.9Example 6 48.3 20.7 30 1.6 15 1350 61 0.98 <0.01 1.6 2.9 Example 7 60 400 1.6 15 1200 48 0.97 <0.01 0.0 2.9 Example 8 41.8 13.2 45 0 15 1300 580.99 <0.01 1.8 0.0 Example 9 41.8 13.2 45 0 15 1350 58 0.98 <0.01 1.20.0 Example 10 25 30 45 0 15 1350 58 0.98 <0.01 3.8 0.0 Comp. Ex. 1 41.813.2 45 1.6 0 1300 10 — — — 2.9 Comp. Ex. 2 41.8 13.2 45 1.6 15 900 660.99 0.08 4.2 2.9 Comp. Ex. 3 90 10 0 1.6 — — 8 — — — 2.9 Comp. Ex. 4petroleum pitch 0 15 1200 — 0.68 0.02 0.0 0 Comp. Ex. 5 petroleum pitch0 2 1200 — 0.65 0.02 0.0 0 Comp. Ex. 6 needle coke 0 — 1200 — 0.71 0.010.0 0 Comp. Ex. 7 phenolic resin 0 — 1200 40 0.97 0.03 1.7 0 Comp. Ex. 8phenolic resin 0 — 1000 43 0.96 0.06 2.1 0 Comp. Ex. 9 41.8 13.2 45 0 161080 — 0.98 0.05 2.6 0

TABLE 2 Specific X-ray diffraction data Bulk Exothermic Particle sizesurface area S d₀₀₂ Lc₍₀₀₂₎ specific peak temp. Dv₅₀ μm D₄/D₁ m²/g S ×Dv₅₀ nm gravity Tp ° C. Example 1 8 1.28 2.8 22.4 0.390 1.0 0.54 622Example 2 9 1.25 1.4 12.6 0.385 1.1 0.55 637 Example 3 7 1.31 1.7 11.90.373 1.2 0.55 639 Example 4 9 1.26 1.0 9.0 0.384 1.1 0.54 637 Example 58 1.33 2.0 16.0 0.383 1.1 0.55 638 Example 6 10 1.23 3.2 32.0 0.375 1.30.56 645 Example 7 8 1.26 >30 >240 0.395 1.1 0.54 640 Example 8 12 1.263.2 38.4 0.380 1.1 0.53 637 Example 9 11 1.25 3.0 33.0 0.374 1.2 0.55660 Example 10 17 1.23 3.6 61.2 0.378 1.1 0.55 664 Comp. Ex. 2 11 1.284.0 44.0 0.405 0.9 0.48 615 Comp. Ex. 4 9 4.42 6.0 54.0 0.380 1.1 0.48640 Comp. Ex. 5 12 4.51 1.5 18.0 0.356 2.3 0.59 735 Comp. Ex. 6 7.8 4.632.5 19.5 0.349 2.5 0.62 760 Comp. Ex. 7 14 1.45 >30 >420 0.386 0.9 0.51650 Comp. Ex. 8 14 1.45 >30 >420 0.395 0.9 0.47 649 Comp. Ex. 9 121.28 >30 >420 0.401 0.9 0.5 623 Electrochemical performances DedopingIrreversible Output Cycle Atmospheric capacity capacity Efficiencyperformance performance degradation mAh/g % mAh/cm² % % Example 1 545121 82 3.8 89 2 Example 2 469 100 82 3.5 90 1 Example 3 458 95 83 3.4 921 Example 4 471 105 82 3.6 90 0 Example 5 465 98 83 3.5 91 1 Example 6434 75 85 3.3 93 2 Example 7 455 110 81 3.4 89 0 Example 8 470 99 83 3.590 6 Example 9 372 85 81 4.2 92 1 Example 10 357 81 82 4.1 93 0 Comp.Ex. 2 592 255 70 3.0 75 7 Comp. Ex. 4 430 80 84 2.3 91 10 Comp. Ex. 5322 98 77 2.2 73 2 Comp. Ex. 6 233 50 82 2.1 <70 1 Comp. Ex. 7 415 13276 3.0 <70 3 Comp. Ex. 8 415 132 76 4.0 <70 5 Comp. Ex. 9 540 165 77 3.284 8

INDUSTRIAL APPLICABILITY

As is apparent in view of the results shown in the above Tables 1 and 2,according to the present invention, there is provided a negativeelectrode material which comprises a spherical carbon material of asmall and uniform particle size, is excellent in quick outputperformance and durability, has a large discharge capacity and istherefore extremely suitable for constituting a negative electrode of anon-aqueous electrolyte secondary battery.

1. A negative electrode material for non-aqueous electrolyte secondarybatteries, comprising: a carbon material having a sphericity of at least0.8, and exhibiting an average (002) interlayer spacing d₀₀₂ of 0.365-0.400 nm, a crystallite size in a c-axis direction Lc₍₀₀₂₎ of 1.0-3.0nm, as measured by X-ray diffractometry, a hydrogen-to-carbon atomicratio (H/C) of at most 0.1 as measured by elementary analysis, and anaverage particle size Dv₅₀ of 1-20 μm.
 2. A negative electrode materialaccording to claim 1, comprising a carbonization product of a vinylresin.
 3. A negative electrode material according to claim 1 or 2,having a bulk specific gravity of at least 0.40 and below 0.60.
 4. Anegative electrode material according to any one of claims 1-3, having aratio D₄/D₁ of at most 3.0 between a weight-average particle size D₄ anda length average particle size D₁.
 5. A negative electrode materialaccording to any one of claims 1-4, having a product of a specificsurface area S(m²/g) and an average particle size Dv₅₀(μm) of 3-40.
 6. Anegative electrode material according to any one of claims 1-5,exhibiting an exothermic peak temperature of at least 600° C.
 7. Anegative electrode material according to any one of claims 1-6,comprising a surface of the carbon material coated with 0.1-10 wt. % ofa silicon compound.
 8. A negative electrode material according to anyone of claims 1-7, containing 0.5-5 wt. % of nitrogen.
 9. A process forproducing a negative electrode material for non-aqueous electrolytesecondary batteries according to any one of claims 1-8, comprising;oxidizing a spherical vinyl resin obtained through suspensionpolymerization to oxidation at a temperature of 150-400° C. in anoxidizing gas atmosphere to provide a carbon precursor and carbonizingthe carbon precursor in an inert gas atmosphere.
 10. A negativeelectrode for non-aqueous electrolyte secondary batteries, having alayer of active substance comprising a negative electrode materialaccording to any one of claims 1-8 and formed at a coating rate of atmost 60 g/m².
 11. A non-aqueous electrolyte secondary battery having anegative electrode according to claim 10.